The present invention is directed to new and useful multi-layer heat shrinkable film formulations. One distinguishing feature of a heat shrink film is the film's ability, upon exposure to a certain temperature, to shrink or, if restrained from shrinking, to generate shrink tension within the film.
The manufacture of shrink films, is well known in the art, may be generally accomplished by extrusion (single layer films) or coextrusion (multi-layer films) of thermoplastic resinous materials which have been heated to their flow or melting point from an extrusion or coextrusion die in, for example, either tubular or planer (sheet) form. After a post extrusion quenching to cool by, for example, the well-known cascading water method, the relatively thick "tape" extrudate is then reheated to a temperature within its orientation temperature range and stretched to orient or align the crystallites and/or molecules of the material. The orientation temperature range for a given material or materials will vary with the different resinous polymers and/or blends thereof which comprise the material. However, the orientation temperature range for a given thermoplastic material may generally be stated to be below the crystalline melting point of the material but above the second order transition temperature (sometimes referred to as the glass transition point) thereof. Within this temperature range it is easy to effectively orient the material.
The terms "orientation" or "oriented" are used herein to generally describe the process step and resultant product characteristics obtained by stretching and immediately cooling a resinous thermoplastic polymeric material which has been heated to a temperature within its orientation temperature range so as to revise the molecular configuration of the material by physical alignment of the crystallites and/or molecules of the material to improve certain mechanical properties of the film such as, for example, shrink tension and orientation release stress. Both of these properties may be measured in accordance with ASTM D 2838-81. When the stretching force is applied in one direction uniaxial orientation results. When the stretching force is applied in two directions biaxial orientation results. The term oriented is also used herein interchangeably with the term "heat shrinkable" with these terms designating a material which has been stretched and set by cooling while substantially retaining its stretched dimensions. An oriented (i.e. heat shrinkable) material will tend to return to its original unstretched (unextended) dimensions when heated to an appropriate elevated temperature.
Returning to the basic process for manufacturing the film as discussed above, it can be seen that the film, once extruded (or coextruded if it is a multi-layer film) and initially cooled to by, for example, cascading water quenching, is then reheated to within its orientation temperature range and oriented by stretching. The stretching to orient may be accomplished in many ways such as, for example, by "blown bubble" techniques or "tenter framing". These processes are well known to those in the art and refer to orientation procedures whereby the material is stretched in the cross or transverse direction (TD) and/or in the longitudinal or machine direction (MD). After being stretched, the film is quickly quenched while substantially retaining its stretched dimensions to rapidly cool the film and thus set or lock-in the oriented (aligned) molecular configuration.
Of course, if a film having little or no orientation is desired, e.g. non-oriented or non-heat shrinkable film, the film may be formed from a non-orientable material or, if formed from an orientable material may be "hot blown". In forming a hot blown film the film is not cooled immediately after extrusion or coextrusion but rather is first stretched shortly after extrusion while the film is still at an elevated temperature above the orientation temperature range of the material. Thereafter, the film is cooled, by well-known methods. Those of skill in the art are well familiar with this process and the fact that the resulting film has substantially unoriented characteristics. Other methods for forming unoriented film are well known. Exemplary, is the method of cast extrusion or cast coextrusion which, likewise, is well known to those in the art.
The degree of stretching controls the degree or amount of orientation present in a given film. Greater degrees of orientation are generally evidenced by, for example, increased values of shrink tension and orientation release stress. That is, generally speaking, for films manufactured from the same material under otherwise similar conditions, those films which have been stretched, e.g. oriented, to a greater extent will exhibit larger values for free shrink, shrink tension and/or orientation release stress. As stated above, the last two values are to be measured in accordance with ASTM-D-2838-81. The first value should be measured in accordance with ASTM D 2732-70 (reapproved 1976).
After setting the stretch-oriented molecular configuration the film may then be stored in rolls and utilized to tightly package a wide variety of items. In this regard, the product to be packaged may first be enclosed in the heat shrinkable material by heat sealing the shrink film to itself where necessary and appropriate to form a pouch or bag and then inserting the product therein and closing the bag or pouch by heat sealing or other appropriate means such as, for example, clipping. If the material was manufactured by "blown bubble" techniques the material may still be in tubular form or it may have been slit and opened up to form a sheet of film material. Alternatively, a sheet of the material may be utilized to over-wrap the product which may be in a tray. These packaging methods are all well known to those of skill in the art. Thereafter, the enclosed product may be subjected to elevated temperatures by, for example, passing the enclosed product through a hot air of hot water tunnel. This causes the enclosing film to shrink around the product to produce a tight wrapping that closely conforms to the contour of the product. As stated above, the film sheet or tube may be formed into bags or pouches and thereafter utilized to package a product. In this case, if the film has been formed as a tube it may be preferable to first slit the tubular film to form a film sheet and thereafter form the sheet into bags or pouches. Such bag or pouch forming methods, likewise, are well known to those of skill in the art.
The above general outline for manufacturing of films is not meant to be all inclusive since such processes are well known to those in the art. For example, see U.S. Pat. Nos. 4,274,900; 4,229,241; 4,194,039; 4,188,443; 4,048,428; 3,821,182 and 3,022,543. The disclosures of these patents are generally representative of such processes and are hereby incorporated by reference.
Alternative methods of producing films of this type are known to those in the art. One well-known alternative is the method of forming a multi-layer film by an extrusion coating rather than by an extrusion or coextrusion process as was discussed above. In extrusion coating a first tubular layer is extruded and thereafter an additional layer or layers is sequentially coated onto the outer surface of the first tubular layer or a successive layer. Exemplary of this method is U.S. Pat. No. 3,741,253. This patent is generally representative of an extrusion coating process and is hereby incorporated by reference.
Many other process variations for forming films are well known to those in the art. For example, multiple layers may be first coextruded with additional layers thereafter being extrusion coated thereon. Or two multi-layer tubes may be coextruded with one of the tubes thereafter being extrusion coated or laminated onto the other. The extrusion coating method of film formation may be preferable to coextruding the entire film when it is desired to subject one or more layers of the film to a treatment which may be harmful to one or more of the other layers. Exemplary of such a situation is a case where it is desired to irradiate one or more layers of a film containing an oxygen barrier layer comprised of one or more copolymers of vinylidene chloride and vinyl chloride. Those of skill in the art generally recognize that irradiation is generally harmful to such oxygen barrier layer compositions. Accordingly, by means of extrusion coating, one may first extrude or coextrude a first layer or layers, subject that layers or layers to irradiation and thereafter extrusion coat the oxygen barrier layer and, for that matter, other layers sequentially onto the outer surface of the extruded previously irradiated tube. This sequence allows for the irradiation cross-linking of the first layer or layers without subjecting the oxygen barrier layer or other sequentially added layers to the harmful effects thereof.
Irradiation of an entire film or a layer or layers thereof may be desired so as to improve the film's resistance to abuse and/or puncture and other physical characteristics. It is generally well known in the art that irradiation of certain film materials results in the cross-linking of the polymeric molecular chains contained therein and that such action generally results in a material having improved abuse resistance. When irradiation is employed to accomplish the cross-linking, it may be accomplished by the use of high energy irradiation using electrons, X-rays, gamma rays, beta rays, etc. Preferably, electrons are employed of at least about 10.sup.4 electron volt energy. The irradiation source can be a Van de Graaff electron accelerator, e.g. one operated, for example, at about 2,000,000 volts with a power output of about 500 watts. Alternatively, there can be employed other sources of high energy electrons such as the General Electric 2,000,000 volt resonant transformer or the corresponding 1,000,000 volt, 4 kilowatt, resonant transformer. The voltage can be adjusted to appropriate levels which may be, for example, 1,000,000 or 2,000,000 or 3,000,000 or 6,000,000 or higher or lower. Other apparatus for irradiating films are known to those of skill in the art. The irradiation is usually carried out at between one megarad and 75 megarads, with a preferred range of 8 megarads to 20 megarads. Irradiation can be carried out conveniently at room temperature, although higher and lower temperatures, for example, 0.degree. C. to 60.degree. C. may be employed.
Cross-linking may also be accomplished chemically through utilization of peroxides as is well known to those of skill in the art. A general discussion of cross-linking can be found at pages 331 to 414 of volume 4 of the Encyclopedia of Polymer Science and Technology, Plastics, Resins, Rubbers, Fibers published by John Wiley & Sons, Inc. and copyrighted in 1966. This document has a Library of Congress Catalog Card Number of 64-22188.
Another possible processing variation is the application of a fine mist of a silicone or anti-fog spray to the interior of the freshly extruded tubular material to improve the further processability of the tubular material. A method and apparatus for accomplishing such internal application is disclosed in a European patent application under publication no. of 0071349A2. This document was published on or about Feb. 9, 1983.
The polyolefin family of shrink films and, in particular, the polyethylene family of shrink films provide a wide range of physical and performance characteristics such as, for example, shrink force (the amount of force that a film exerts per unit area of its cross-section during shrinkage), the degree of free shrink (the reduction in linear dimension in a specified direction that a material undergoes when subjected to elevated temperatures while unrestrained), tensil strength (the highest force that can be applied to a unit area of film before it begins to tear apart), heat sealability (the ability of the film to heat seal to itself or another given surface), shrink temperature curve (the relationship of shrink to temperature), tear initiation and tear resistance (the force at which a film will begin to tear and continue to tear), optics (gloss, haze and transparency of material), elongation (the degree the film will stretch or elongate at room temperature), elastic memory (the degree a film will return to its original unstretched (unelongated) dimension after having been elongated at room temperature), and dimensional stability (the ability of the film to retain its original dimensions under different types of storage conditions). Film characteristics play an important role in the selection of a particular film and they may differ for each film application.
In view of the many above-discussed physical characteristics which are associated with polyolefin films and films containing a polyolefin constituent and in further view of the numerous applications with which these films have already been associated and those to which they may be applied in the future, it is readily discernable that the need for ever improving any or all of the above described physical characteristics or combinations thereof in these films is great, and, naturally, ongoing. In particular, the quest for highly oriented films which may be utilized as packaging materials is ever ongoing. One very desirable trait that a shrink film should possess is improved shrinkage (percent free shrink) at lower temperatures. This allows the hot air tunnels or hot water baths which are utilized in the industry to shrink a bag or package formed from the film into close configuration with the packaged article to be operated at lower temperatures and results in energy savings. Another desirable trait a shrink film should have is a relatively gradual and linear shrink/temperature curve. The shrinkage a given film will undergo when heated to a given temperature can be more acurately estimated and controlled when the shrink/temperature curve (ratio of free shrink versus degrees Fahrenheit) of a film is approximately gradual and linear. Yet another trait a shrink film should have is good abuse resistance. This trait is generally evidenced by improved ball burst and tear propagation values.
Prior art films utilizing linear polyethylene materials and blends thereof are known to those of skill in the art. Exemplary multilayer prior art films having a core layer of linear low density polyethylene material are U.S. Pat. No. 4,364,981 to Horner which discusses a three layer film having a core layer of low pressure, low density polyethylene (LLDPE) and outer layers of high pressure, low density polyethylene (conventional low density polethylene) and U.S. Pat. No. 4,399,180 to Briggs which discusses a stretch-wrap film having a core layer of linear low density polyethylene with a layer, on at least one side, comprising a highly branched low density polyethylene. U.S. Pat. No. 4,399,173 to Anthony discusses a multilayer film comprising a core layer of low melt index, low pressure, low density polyethylene and two outer layers of a high melt index, low pressure, low density polyethylene. U.S. Pat. No. 4,425,268 to Cooper discloses a composition adapted for processing into stretch-wrap film. Generally, the Cooper composition comprises a blend of an ethylene vinyl acetate copolymer and a linear low density polyethylene material. The material may also contain a tackifier.
An exemplary prior art monolayer film is a monolayer film comprising a blend of (a) about 50%, by weight, of a conventional low density polyethylene resin having a density of about 0.922 grams per cubic centimeter at 23.degree. C. and a melt flow index of about 1.8-2.0 grams per ten minutes (ASTM D 1238, condition E) and (b) about 50%, by weight, of a conventional high density polyethylene resin having a density of about 0.953 grams per cubic centimeter and a melt flow index of from about 5.4 to about 7.4 grams per ten minutes. The lower density resin may be obtained from the El Paso Polyolefins Company under the trade designation of Rexene PE-109-CS-52 resin. The high density material may be obtained from the U.S. Industrial Chemicals Co. under the trade designation Petrothene LY660 resin. The monolayer film may also contain appropriate antioxidants and other additives, as necessary. This prior art film will hereafter be referred to as prior art film "A".
Another exemplary prior art monolayer film is a film comprising about 100%, by weight, of a linear medium density polyethylene material having a density of about 0.935 grams per cubic centimeters at 23.degree. C. and a melt flow index of about 2.55 grams per ten minutes (ASTM D 1238, condition E). This material may be obtained from the Dow Chemical Company undet the trade designation Dowlex 2037 resin. The film may contain small amounts of additives and, preferably, comprises a surface coating of a polyorganosiloxane material. The surface coating is a processing aid applied to the inner surface of the material shortly after its extrusion in tubular form. This prior art monolayer film will hereafter be referred to as prior art film "B".